1. Technical Field
The disclosure generally relates to a prioritized random access method for wireless communication devices with different priority levels.
2. Related Art
Machine to Machine (M2M) communications (also called machine-type-communication, abbreviated as MTC) is a very distinct capability that enables the implementation of the “Internet of things”. It is defined as information exchange between a subscriber station (or a wireless communication device) and a server in the core network (through a base station) or just between subscriber stations, which may be carried out without any human interaction. Several industry reports have scoped out huge potential for this market. Given the huge potential, some novel broadband wireless access systems, such as 3GPP LTE and IEEE 802.16m, have started to develop enhancements for enabling M2M communications.
In some use case models of M2M communications, such as healthcare, secured access & surveillance, public safety, and remote maintenance & control, high priority access is necessary in order to communicate alarms, emergency situations or any other device states that require immediate attention. Besides, for battery-limited M2M devices, consuming extremely low operational power over long periods of time is required. Such M2M devices may be in idle mode at most time for power saving. Hence, prioritized ranging (or random access) is an essential function for idle M2M devices while they want to transmit delay-sensitive messages to the M2M server(s). On the other hand, in such urgent cases, the backbone wireless communication system should have ability to provide enough ranging capacity for those delay-sensitive applications even if it may be a rare case of mass ranging attempts for emergency occurring simultaneously.
According to current IEEE 802.16m specification, when an idle mobile station wants to perform network entry, it will receive the system information carried in a superframe header at first for knowing the current configuration of ranging code partition. The Code Division Multiple Access (CDMA) ranging codes are divided into initial ranging codes and handover ranging codes. After obtaining the current configuration of ranging code partition, the mobile station randomly selects an initial ranging code to perform CDMA ranging. If the base station detects the ranging code, the base station responds by a MAC (Medium Access Control) control message, AAI_RNG-ACK, to indicate whether the CDMA ranging is successful or not. If the CDMA ranging is successful, the base station allocates uplink (UL) bandwidth to the mobile station subsequently. Then, the mobile station uses the allocated bandwidth to transmit AAI_RNG-REQ and waits for the corresponding response message, AAI_RNG-RSP.
As in the aforementioned description, it is known that there is no design for prioritized ranging (or random access) in IEEE 802.16m currently. However, it may result in contentions and collisions during network entries between an emergency access and a normal access. If an idle M2M device performs CDMA ranging for transmitting delay-sensitive message with the same ranging opportunity used by the mobile station (i.e., using the same code at the same time and frequency resource), the base station cannot detect that the same ranging code(s) were transmitted from two different devices. Therefore, after receiving AAI_RNG-ACK, the mobile station and the M2M device assume their CDMA ranging is successful and then the following AAI_RNG-REQ messages transmitted from these two devices will collide. Subsequently, the base station may recognize one of these two AAI_RNG-REQ messages or lose the both two. If a mobile station does not receive the corresponding AAI_RNG-RSP message, it will perform CDMA ranging again. As such, time consumed by random access and contention resolution is a dominant time during network entry.
The random access procedure in 3GPP LTE specification is similar to the aforementioned ranging procedure in IEEE 802.16m. 3GPP LTE additionally introduces a mechanism, called Access Class Barring (ACB), to configure different barring factors and barring times for each access class. If ACB is configured, an user equipment (UE) should draw a random number uniformly distributed in the range 0 to 1 before performing random access. If the random number is greater than the barring factor associated to the UE, the UE considers random access as barred and postpones its access attempt until a timer expires, where the timer is calculated from the barring time associated to the UE. Although ACB can differentiate the access probabilities of different access classes, it can not guarantee that a high priority random access is served earlier than a low priority random access while contention occurs. Therefore, it is a major concern to modify the conventional random access protocols so as to achieve prioritized random access with congestion detection and contention resolution.